When giving recipients in a string format ("email1, email2") to sendmail(), an email is sent to the first email in the list only.
This is not a bug since the documentation indicates that sendmail() takes either a list of addresses, or a single address in a string. However, this error is easy to make since the "To:" field expects a comma-separated series of email addresses in a string format.

I suggest either that:
- sendmail() accepts a series of emails in a string format: "email1, email2" for the recipient lists
- sendmail() raises an exception if a series of email in a string format is detected

Attached is an example script to test confusing behaviour (email is sent to email1@mydomain.com only):